Micro-electro-mechanical systems ultra-sensitive accelerometer with independent sensitivity adjustment

ABSTRACT

An accelerometer is based upon the monolithic integration of a Fabry-Perot interferometer and a p + n silicon photosensor. Transmission of light through a Fabry-Perot interferometer cavity is exponentially sensitive to small displacements in a movable mirror due to an applied accelerating force. The photosensor converts this displacement into an electrical signal as well as provides for additional amplification. Because the interferometer and photosensor are monolithically integrated on a silicon substrate, the combination is compact and has minimal parasitic elements, thereby reducing the accelerometer&#39;s noise level and increasing its signal-to-noise ratio (SNR). 
     The accelerometer&#39;s sensitivity can be user-controlled by any one or a combination of factors: adjusting the length between the mirrors of the Fabry-Perot cavity; adjusting the power of the light projected to the photosensor; and pulsing the light at a selected frequency to minimize 1/f inherent system noise in the response of the accelerometer.

RELATED APPLICATION

This application is a continuation-in-part of commonly assigned U.S.patent Ser. No. 09/892,301 filed Jun. 26, 2001 now U.S. Pat. No.6,546,798 by inventors Richard L. Waters and Monti E. Aklufi. Thisrelated patent is incorporated by reference herein.

BACKGROUND OF THE INVENTION

This invention relates generally to perceiving acceleration upon anobject. More specifically the invention relates to the devices used formaking such perceptions, known as accelerometers and particularly anoptical accelerometer created through the technology known asmicro-electro-mechanical systems or“MEMS”. With yet greater exactness,the invention relates to adjusting the sensitivity of such anaccelerometer by selectively and independently positioninginterferometers mirrors utilized in the invention.

Micro-electro-mechanical systems use microelectronic processingtechniques wherein mechanical devices arc reduced to the scale ofmicroelectronics. These processing techniques enable the integration ofboth mechanical and electrical components onto a single chip, typicallymade of silicon. Prior to MEMS, accelerometer components were for themost part manufactured separately. These components were then assembledtogether in a process that could easily be complex and expensive.

Current MEMS accelerometer designs have numerous advantages over theirconventional counterparts. The MEMS accelerometers are of small size,light weight and low cost. Their sensitivity, however, has fallenlargely in the low performance regime. Because of their relative lowsensitivity and cost, current MEMS accelerometers have been usedprimarily in the automobile industry as collision airbag sensors and forother low sensitivity applications. Although the collision airbag sensormarket is significant, it is but a small fraction of the potentialmarket for low cost ultra-sensitive MEMS accelerometers.

Existing MEMS accelerometer technology is based upon either capacitiveor piezo-based designs. State of the art MEMS capacitive accelerometersmeasure the charge on a capacitor to detect small movements of a proofmass attached to a spring. However, in order to detect sub milliG (1G=9.8 m/s²) perturbation forces with this technique, elaborateamplification circuitry capable of measuring on the order of nanovoltchanges in potential is necessary. For example, typical steady statecapacitance values for MEMS accelerometers are in the 100 fFarad range,where 1 f−10⁻¹⁵. Furthermore, a 1 G accelerating force results in aminute change in capacitance, on the order of 100 aF where a=10⁻¹⁸. Thisis equivalent to sensing a change of 625 electrons across the plates ofa capacitor at an applied bias of 1 volt. Alternatively stated, this iscommensurate with detecting the presence/absence of approximately 1 outof every 1000 electrons. To amplify this small change in capacitanceextremely sensitive circuitry is required to translate the capacitancevariations into a detectable voltage output signal. Even with theaddition of low noise amplification circuitry, these MEMS accelerometersdo not have the sensitivity required for many potential applications.

Piezoelectric or piezoresistive materials produce either a potentialdifference or a change in resistance when an external pressure/force isapplied. This property lends itself to accelerometer designs. Ashortcoming of piezoelectric or piezoresistive materials is that theyare also pyroresistive, meaning that they change resistance withtemperature. High sensitivity piezo-based accelerometers are thereforedifficult to maintain. In addition, the resistance or change inpotential of such accelerometers is usually extracted from a largeresistor fabricated in the material. This large resistance leads toincreased noise, e.g. resistive noise/Johnson noise. These problems aresignificant for piezo-based accelerometers. More commonly usedaccelerometers therefore use the capacitive method—which also suffersfrom noise but not as severely.

To realize the full potential of MEMS accelerometers, a significantimprovement in sensitivity is required. Ideally, this improvement willminimize accelerometer inherent noise. Possible applications of such lowcost, light weight, ultra-sensitive MEMS accelerometers includebio-mechanics, seismology, condition monitoring of machines andstructures, and robotics. In addition, an ultra-sensitive MEMSaccelerometer would dramatically improve the accuracy of guidance,navigation, and global positioning systems (GPS) that requiresensitivity not on the order of the 1 G scale but rather the on theorder of the μG scale or better.

The invention has structural similarities to an optical switch andamplifier described in the article titled: “Micromechanical OptoelectricSwitch and Amplifier (MIMOSA)” by R. Waters et al, IEEE Journal ofSelected Topics in Quantum Electronics, 5, 33 (January/February 1999)incorporated by reference herein.

SUMMARY OF THE INVENTION

The invention is an improvement upon an accelerometer based upon themonolithic integration of a Fabry-Perot interferometer and a p⁺n siliconphotodiode. The transmission of light through a Fabry-Perot etalon isexponentially sensitive to small displacements in the position of amovable mirror due to changes in an applied accelerating force. Thephotosensor converts this displacement to an electrical signal as wellas provides for additional amplification. Because both the Fabry-Perotmodulator and photodiode are monolithically integrated on a siliconsubstrate, the combination is compact and has minimal parasiticelements, thereby reducing the accelerometer's noise level andincreasing its signal-to-noise ratio (SNR).

The sensitivity of the invention is user-controlled based upon any oneor a combination of factors: adjusting the length between the mirrors ofthe Fabry-Perot etalon; adjusting the power of the light projectedthrough the mirrors to the photodiode; and activating and deactivatingthe light at a selected frequency to minimize 1/f inherent system noisein the response of the accelerometer.

In the present design, the length between the mirrors of the Fabry-Perotetalon is adjustable by a mechanism lying outside of the optical path ofthe photodiode and that does not utilizing the photodiode itself as aconductor to aid in the adjustment of the mirrors.

The MEMS accelerometer of the invention is calculated to be capable ofproducing 1 V/G without the use of amplification circuitry. It isestimated that when amplification circuitry is used with the novel MEMSaccelerometer of the invention, it will be more than three orders ofmagnitude more sensitive than present MEMS accelerometers usingamplification circuitry. This implies that the μG sensitivity needed fornavigation and GPS applications is obtainable if voltage levels on theorder of 1 μV are detectable.

In opposition to prior art designs, the invention uses a light sourcerather than capacitive or piezo-based techniques for sensingacceleration. The advantages of this include use of a small wavelengthof light for accurately measuring the movement of a suspended inertialmass and utilizing the wave nature of light for creating anexponentially sensitive accelerometer that is more than three orders ofmagnitude more sensitive than the previous art.

An object of this invention is to provide an accelerometer of highsensitivity.

Another object of the invention is to provide an accelerometer of highsensitivity in which inherent (1/frequency) noise is minimized.

A further object of this invention is to provide an opticalaccelerometer of high sensitivity.

Still another object of this invention is to provide an opticalaccelerometer in which light power is varied to adjust theaccelerometer's sensitivity.

Still yet another object of this invention is to provide an opticalaccelerometer in which light power is varied to adjust theaccelerometer's sensitivity by decreasing system inherent noise.

Still a further object of this invention is to provide an opticalaccelerometer in which light is selectively pulsed to adjust theaccelerometer's sensitivity.

Still yet a further object of this invention is to provide an opticalaccelerometer that includes a Fabry-Perot etalon in which the distancebetween the etalon's mirrors is adjusted to adjust the accelerometer'ssensitivity.

Yet another object of the invention is to provide an opticalaccelerometer that includes a Fabry-Perot etalon in which the distancebetween the etalon's mirrors is adjusted to adjust the accelerometer'ssensitivity and in which this adjustment is done by a mechanism lyingoutside the light path of the accelerometer and that does not employ thelight sensing mechanism to effectuate the mirror adjustment.

Yet still a further object of this invention is to provide an opticalaccelerometer of high sensitivity that is fabricated throughmicro-electro-mechanical system (MEMS) processing.

Other objects, advantages and new features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side-section view of an accelerometer according to oneembodiment of the invention described by the inventors in theirabove-cited patent.

FIG. 2 illustrates an exemplary top view of an embodiment of theinvention described by the inventors in their above-cited patent.

FIG. 3 illustrates an exemplary top view of another embodiment of theinvention described by the inventors in their above-cited patent.

FIG. 4 illustrates an exemplary top view of yet another embodiment ofthe invention described by the inventors in their above-cited patent.

FIG. 5 describes graphically the relationship between light transmissionand interferometer gap distance as described by the inventors in theirabove-cited patent.

FIG. 6 shows a side-section view of the accelerometer of FIG. 1 modifiedby a weight added to its upper mirror as described by the inventors intheir above-cited patent.

FIG. 7 shows a generalized wiring diagram used in conjunction with theembodiment of the invention of FIG. 1 as described by the inventors intheir above-cited patent.

FIG. 8 depicts yet another embodiment of the invention as described bythe inventors in their above-cited patent.

FIG. 9 shows a further embodiment of the invention as described by theinventors in their above-cited patent.

FIG. 10 illustrates another embodiment of the invention incorporating alight emitting diode as described by the inventors in their above-citedpatent.

FIG. 11 is like FIG. 1 but illustrates placement of a mirror conductoraccording to one embodiment of the present invention.

FIG. 12 illustrates one embodiment of a mirror conductor as may be usedwith the present invention.

FIG. 13 illustrates one embodiment of a mirror conductor as may be usedwith the present invention.

FIG. 14 is like FIG. 7 but illustrates placement of a mirror conductorand its generalized wiring diagram according to an embodiment of thepresent invention.

FIG. 15 is like FIG. 6 but illustrates placement of a mirror conductoraccording to another embodiment of the present invention.

FIG. 16 is like FIG. 8 but illustrates placement of a mirror conductoraccording to another embodiment of the present invention.

FIG. 17 is like FIG. 10 but illustrates placement of a mirror conductoraccording to another embodiment of the present invention.

FIG. 18 illustrates placement of a mirror conductor according to anotherembodiment of the present invention.

FIG. 19 is like FIG. 16 but further illustrates placement of a secondmirror conductor and its generalized wiring diagram according to anembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a micro-electro-mechanical system ultra-sensitiveaccelerometer (MEMSUSA) 10 as described by the inventors in theabove-cited patent is shown by way of example. Accelerometer 10 lendsitself to being made according to well-understood steps familiar to thesemiconductor processing field and the MEMS world. Further descriptionof this processing, therefore, will not be described here.

Accelerometer 10 utilizes a monochromatic light source 12 such as afixed wavelength solid state laser, a light emitting diode or a verticalcavity surface emitting laser (VCSEL). This light is coupled directly orindirectly, such as via fiber-optic cable, to an interferometer 14.Interferometer 14, in this example, is a Fabry-Perot cavity 16.

This Fabry-Perot cavity is the optical cavity between upper and lowermirrors. In this case, a first or upper mirror 18 of the cavity isformed on the top surface of a hinged membrane 20 that is flexiblysuspended above and substantially parallel to a second or lower mirror22. Upper mirror 18 is designed to partially reflect and partiallytransmit light from and into cavity 16, such as may be accomplished by athin semi-transparent mettalization on the top surface of membrane 20.Lower mirror 22 exists on the surface of a p⁺ region 24 created insubstrate 26 which is for example of silicon. Mirror 22 can be made forexample by the semiconductor/air interface or via the deposition of athin semi-transparent metal on the surface of region 24. Both mirrors 18and 22 can be fabricated through the deposition of various dielectriclayers, known as a dielectric stack, to form a dielectric mirror at adesired wavelength. In addition upper mirror 18 can have a thinconducting layer deposited either between the layers of the dielectricstack or on top of the stack to form an electrode for electrostaticactuation.

A p⁺n junction 28 creates a photodiode used to absorb light 30.Substrate 26 is disposed upon an n⁺ substrate contact 32. The p⁺ regionreaches a metal contact 34 via a path not shown but within substrate 26.Of course, photodiodes of other configurations may be used, such as n⁺p,pin and Schottky diode, for example.

Operation of the proposed device can be understood by examining both thetransmission of light through the Fabry-Perot etalon for a fixed mirrorspacing and for a change in mirror spacing such as will occur with anapplied external force. A maximum in the transmission of light throughthe Fabry-Perot cavity, with monochromatic light incident normal to thesurface of the mirrors, is achieved if the distance between the mirrorsis an integral multiple of half wavelengths of the light. A maximum inlight transmission through the cavity implies a maximum in thephoto-generated current in the underlying photodiode of the structure.Further, one of the two mirrors of the Fabry-Perot interferometer ismade such that it is hinged and therefore not rigidly fixed in position.In the example shown, the upper mirror is flexibly supported by foursymmetrically located silicon dioxide beams 36.

Two configurations of beam location for square and circular structuresarc shown in FIGS. 2 and 3, respectively. FIG. 4 illustrates multilevels of support beams. FIGS. 2-4 represent the top view of partiallymetallized upper mirrors which are deposited on a hinged membranesupported by support beams. The material used to fabricate the membraneand support legs need not be the same material and their size andgeometry including the number of legs may change to adjust thesensitivity of the hinged mirror to applied external forces.

As illustrated graphically in FIG. 5, applied forces due to accelerationor an electrostatic attraction between the mirrors will change theeffective optical path traversing the cavity length between the mirrorscausing an exponential change in the transmission of light into thephotodiode. Depending upon the design of the structure, i.e. membranematerial and thickness and cavity length (air gap) distance, multiplepeaks and valleys of the transmission may occur as an external force isapplied. Each peak/valley in the transmission corresponds to a differentrange of sensitivity due to a change in the effective spring constant.The “circles” in this graph show airgap (cavity length) distances ofmaximum sensitivity for the wavelength of light used. Thus, as will bediscussed further, an applied bias across the mirrors can be used totune the sensitivity range. Adjusting materials and geometries duringthe fabrication process can also be used to adjust the range ofsensitivities.

Referring to FIG. 6, another embodiment of this invention is shown inwhich metal weights 38 are patterned on the upper cavity mirror toprovide an additional inertial mass to this movable mirror. The weightsin this instance also serve to restrict light to only the p-n junctionregion, thus decreasing the steady-state light absorbed and thereforeincreasing the on/off ratio of the sensor. Of course, one will realizethat other weight material besides metal may be used in thisapplication. For example, wafer bonding techniques may be used toprovide one or more silicon weights as inertial mass for the invention.Other weight materials and other techniques of applying/attaching weightmaterials are also of course possible.

FIG. 7 illustrates one possible biasing configuration in whichsensitivity of the accelerometer can be surmised. In this figure, aconstant current is applied through the photodiode while the voltage,Vsensitivity, is adjusted to maintain a constant force on the uppermirror, thus insuring a constant desired air gap (cavity length d) andphotodiode current. The control voltage, Vsensitivity, is then comparedto a reference voltage (Vref) and the difference is amplified andfiltered. In this simplified configuration, the output voltage can bedirectly related to the force applied perpendicularly to the uppermirror. Hence a change in the output voltage can be used as a measure ofthe acceleration force.

FIG. 8 is yet another embodiment of the invention shown in this casewith exemplary materials described. To further increase the inertialmass for high sensitivity applications, a lower cavity mirror includinga p⁺n photodiode, is flexibly supported by cantilevered beams, 40. Theupper cavity mirror in this embodiment is rigidly supported.

Alternatively, FIG. 9 shows a top mirror made from a bulk silicon wafer.In this alternative configuration of the accelerometer, two wafers 42and 44 are bonded together by heat or pressure. Wafer 42 defines anupper mirror configuration and supporting legs/springs. These supportsmay be of the same composition as the upper mirror or of a differentcomposition. In this embodiment, backside etching through wafer 42 isused to define an optical opening through which monochromatic light maybe illuminated onto the upper mirror. Wafer 44 defines the lower mirrorconfiguration as well as a spacer used to separate the two mirrors.Wafer 44 also includes a diffused p-type region under the lower mirror.The thickness of the spacer of wafer 44 together with the thickness ofthe supporting legs/springs of wafer 42 define the effective opticalcavity length “d” of this configuration. In this embodiment, wafer 42'ssubstrate gives an additional inertial mass to the upper mirror therebyincreasing its displacement for a given acceleration and increasing thesensitivity of the accelerometer.

Depending upon the design and geometry of the sensor as discussed above,a number of methods can be used to bring light into the structure. Thesemethods include epoxy bonding a solid state laser or light emittingdiode to a packaged optical accelerometer possessing a clear opticalwindow. Another choice is to wafer bond vertical cavity surface emittinglasers (VCSEL) such that they are suitably positioned over the opticalcavity. In addition, FIG. 10 illustrates how organic LEDs 46 can bedeposited on a rigidly attached transparent membrane. This embodiment isshown with exemplary materials identified. Fiber optic cables can alsobe used to bring light into the structure. The fiber optic cables may bealigned to the top mirror of the membrane by any suitable alignmenttechniques such as selectively etching alignment marks in the silicon oretching precision openings in the bulk silicon to allow the fiber opticcore to align with the top mirror, etc.

In all of the embodiments, Pulse Width Modulation (PWM) of the lightused can be performed to avoid the characteristic one over frequencynoise (1/f) that plagues current accelerometers, thereby enhancing thesensitivity of the accelerometer. Instead of having a constant intensitylight source, the source will be pulsed with some characteristicfrequency, fl, such that the electrical signal generated by theaccelerometer is at fl plus the maximum response of the accelerometer.Since the 1/f noise decreases with frequency, shifting the accelerometerresponse to a higher frequency will reduce the noise from this source.The 1/f noise is inherent in all accelerometers and electronic circuits,it can not be avoided by simple mechanical means.

A unique advantage of the invention is its ability to vary accelerometersensitivity level by adjusting input light intensity. In the quantummechanical limit, photodiode noise (shot noise) increases linearly withthe square root of the input light power. At the same time, however, thedetected electrical signal in the photodiode varies linearly with theinput light power. Therefore, the ratio of the detected signal to thenoise generated within the photodiode increases as the square root ofthe input power. This increasing signal to noise ratio (SNR) withincreasing optical power allows one to adjust or increase thesensitivity of the accelerometer by increasing the light power incidentupon the photosensor.

As previously described, the invention also lends itself to sensitivityadjustment via changing of the distance between the accelerometer'smirrors. This distance is easily altered by applying a selectedpotential across the mirrors.

Though the invention has been described in terms of Silicon, similarstructures can be fabricated in a material system other than Silicon,such as Indium Phosphide (InP) or Galium Arsenide (GaAs), for example.These will also allow monolithic integration of a light source with aphotodiode and membrane structure. In cases where applicable, upper andlower mirrors may be fabricated on separate wafers by ionic, heat orpressure bonding.

Referring now to FIG. 11, the present invention will be described. Thepresent invention is an improvement upon the above-describedaccelerometer design. As described above, one mechanism for enhancingthe sensitivity of the MEMS accelerometer is for the user to adjust thecavity length (air gap) between the accelerometer's interferometermirrors.

In the previously described embodiment of the invention, this adjustmentis accomplished by providing a potential between the photodiode and theflexibly suspended mirror. By generating an electrostatic attractionbetween these elements, it is possible to cause the two interferometermirrors to converge. While this method of accelerometer sensitivityadjustment has its merits, there are some drawbacks that in cases willdetract from accurate accelerometer readings.

It has also been realized that under certain conditions current absorbedin the photodiode photosensor of the invention generates a voltage thatcan adversely affect accurate relative positioning of the mirrors. It isalso desirable at times to bias the photodiode photosensor and this inturn can also adversely affect the relative positioning of theinterferometer mirrors.

In FIG. 11, a grid electrode 50 is shown in this cross-section aslocated generally between the springs/support structure 52 of firstmirror 54 and photosensor 56. As can be seen in this figure, electrode50 is disposed outside a path 58 of light source 60. FIGS. 12 and 13illustrate two variations on how the grid electrode may be patterned. Ascan be seen in FIG. 12, grid electrode 50 takes the shape of aconcentric ring, shown in this figure further from the viewer thanmirror 54 and supports 52. A path is provided through this ring so thatphotosensor 56 can be illuminated through the central void defined bythe electrode. In FIG. 13, mirror 54 and accompanying supports 52 areclosest to the viewer while electrode 50, not shown in this figure, isdisposed directly underneath supports 52. Of course, other geometricalpatterns and variations for the grid electrode exist that may allow forincreased sensitivity and/or control of the suspended mirrordisplacement.

Referring now to FIG. 14, to provide selective mirror positioning, thegrid electrode may be maintained at a constant voltage and the voltageon the flexibly suspended mirror changed, or the voltage on the flexiblysuspended mirror is kept constant and the voltage on the grid electrodechanged.

The grid electrode may be used in a force-rebalance mode to decouplefeedback from the accelerating force sensing element (in this caseflexibly suspended mirror 54). The force rebalance mode keeps the totalforce on this mirror constant such that, the force due to inputacceleration plus the capacitive force exerted by the grid electrode onmirror 54 is constant. This is achieved by applying a variable potentialto the grid electrode thus adjusting the capacitive force as the inputaccelerating force changes, resulting in a constant total force applied.This also prevents stiction of the upper mirror to the lower mirrorwhich has been reported to be a common problem for the prior art.

The grid electrode also shields the upper (first) mirror from anycapacitive force exerted by the photosensor substrate. The capacitiveforce from this substrate is undesirable because its potential and henceforce may change as the acceleration changes and hence collected currentin the photodiode changes. Depending upon the point of operation thisfeedback can be either positive or negative.

The grid electrode allows not only the initial cavity length between theinterferometer mirrors to be adjusted but also allows adjustment of theeffective spring constant of the mirror support structure. The abilityto independently adjust the spring constant allows the dynamic range ofthe accelerometer to be adjusted.

FIGS. 15-17 show alternative arrangements of utilizing such an electrodewith many of the embodiments of the accelerometers described above. FIG.18 illustrates an optical sensor wherein electrode 50 is encapsulatedunderneath mirror 62 and within a region below the spring/supportstructure of mirror 54.

FIG. 19 shows another embodiment of the invention in which an electrode64 is disposed to draw mirror 54 away from mirror 62. Electrode 64 maybe effectuated via wafer bonding a suitable substrate 66 with patternedelectrodes. Use of such an “upper” electrode is possible in conjunctionwith the previously described grid electrode, as is shown in FIG. 19.One advantage of this technique, among others, is the ability to adjustthe interferometer cavity length in both positive and negativedirections.

There are numerous advantages in using control electrodes in conjunctionwith an accelerometer as described herein.

The invention provides independent control of the accelerometer's springconstant. By varying the spring constant, the dynamic range of aparticular accelerometer sensor may be adjusted. Identicalaccelerometers may be arrayed such that the spring constant for each isadjusted. In this manner, the overall dynamic range of the compositeaccelerometer array is increased over that of an individualaccelerometer sensor.

By using the invention, it is possible to decouple feedback from theaccelerometer's force sensing element, i.e. feedback control is obtainedvia the control electrode as opposed to a voltage directly on thesensing element.

The invention permits adjustment of the accelerometer's cavity length toaccommodate tuning to an optimal operating point.

The invention allows for the local adjustment of the potentials oneither side of the photosensor diode junction, e.g. the potentials onthe p+ and n regions of the photodiode can be adjusted without affectingthe net force that the force sensing element feels. This ability toadjust the potentials across the photodiode also increases flexibilityin the design of control/interface electronics.

In the event that an unwanted spurious external/internal force acts uponthe force-sensing element of the accelerometer, the control electrodecan be used to apply a force equal and opposite to counter balance (orforce rebalance) the system and adjust the net force to zero. In the“force rebalance mode” described, the control electrode helps to preventstiction of one interferometer mirror to the other while simultaneouslydecoupling the interferometer cavity length adjusting force from theaccompanying photosensor.

A further advantage of the invention is that field lines generated bythe photosensor substrate terminate on the control electrode therebyshielding these from the upper interferometer mirror, thereby decouplingmovement of this mirror from the bias on the substrate.

The control electrodes described arc not meant to be directed towards asingle “lower” and a single “upper” electrode. One may incorporate aseries or matrix of electrodes disposed at either location and stillfall within the spirit of the invention and described herein.

Though the invention has been described as a sensor for sensingacceleration, it is by no means limited to this particular application.It can be envisioned that various other force sensing applications ofthe invention are possible. Included among these, but withoutlimitations thereto, are sensing application in the inertial, magnetic,pressure, electrostatic and thermal areas for example.

In addition, the invention has been largely described by way of exampleto employ a monochromatic light source. The invention is however alsoconsidered usable with light of a plurality of colors or wavelengths.

Obviously, many modifications and variations of the invention arepossible in light of the above description. It is therefore to beunderstood that within the scope of the claims the invention may bepracticed otherwise than as has been specifically described.

What is claimed is:
 1. An apparatus comprising: a pair of partiallytransmissive, partially reflective, surfaces wherein a first of saidsurfaces is flexibly suspended adjacent and substantially parallel to asecond of said surfaces, said surfaces defining a cavity and a cavitylength therebetween; a photosensor attached to one of said surfacesoutside of said cavity; a source of light, said light for irradiatingsaid photosensor through said first and second surfaces wherein saidlight is also partially reflected between said surfaces; and a cavitylength adjustor independent of said photosensor for adjusting saidcavity length, wherein a change in said cavity length is sensed by achange in said light as detected by said photosensor.
 2. The apparatusof claim 1 wherein said cavity length adjustor includes an electrodedisposed to bias said first partially transmissive, partially reflectivesurface towards said second partially transmissive, partially reflectivesurface.
 3. The apparatus of claim 1 wherein said cavity length adjustorincludes an electrode disposed to bias said first partiallytransmissive, partially reflective surface away from said secondpartially transmissive, partially reflective surface.
 4. The apparatusof claim 2 wherein said cavity length adjustor includes an electrodedisposed to bias said first partially transmissive, partially reflectivesurface away from said second partially transmissive, partiallyreflective surface.
 5. The apparatus of claim 2 wherein said bias isapplied between said electrode and said first partially transmissive,partially reflective surface.
 6. The apparatus of claim 2 wherein saidbias is applied between said electrode and a flexible support of saidfirst partially transmissive, partially reflective surface.
 7. Anapparatus comprising: an interferometer, including pair of partiallytransmissive, partially reflective, surfaces wherein a first of saidsurfaces is flexibly suspended adjacent and parallel to a second of saidsurfaces, said surfaces defining an interferometer cavity and aninterferometer cavity length therebetween; a photosensor attached tosaid second surface outside of said cavity; a source of monochromaticlight, said light for irradiating said photosensor through said firstand second surfaces wherein said light is also partially reflectedbetween said surfaces; and a cavity length adjustor independent of saidphotosensor for adjusting said cavity length, whereby a change in saidcavity length is sensed by a change in said light as detected by saidphotosensor and wherein sensitivity of said apparatus is variable byadjusting said cavity length and is variable by activating andde-activating said light at a selected frequency.
 8. The apparatus ofclaim 7 wherein said interferometer and said photosensor aremonolithically integrated on a single substrate.
 9. The apparatus ofclaim 7 wherein said cavity length adjustor includes an electrodedisposed so that conduction therethrough biases said first partiallytransmissive, partially reflective surface towards said second partiallytransmissive, partially reflective surface.
 10. The apparatus of claim 7wherein said cavity length adjustor includes an electrode disposed sothat conduction therethrough biases said first partially transmissive,partially reflective surface away from said second partiallytransmissive, partially reflective surface.
 11. The apparatus of claim 9wherein said cavity length adjustor includes an electrode disposed sothat conduction therethrough biases said first partially transmissive,partially reflective surface away from said second partiallytransmissive, partially reflective surface.
 12. The apparatus of claim 9wherein said photosensor is a photodiode having a substrate and whereinsaid electrode shields said first partially transmissive, partiallyreflective surface from capacitive forces exerted by said substrate. 13.The apparatus of claim 9 wherein said bias is applied between saidelectrode and said first partially transmissive, partially reflectivesurface.
 14. The apparatus of claim 7 wherein said bias is appliedbetween said electrode and a flexible support of said first partiallytransmissive, partially reflective surface.
 15. A high sensitivitymicro-electromechanical optical accelerometer comprising: a Fabry-Perotinterferometer, including pair of partially transmissive, partiallyreflective, surfaces wherein a first of said surfaces is flexiblysuspended adjacent and parallel to a second of said surfaces, saidsurfaces defining an interferometer cavity and an interferometer cavitylength therebetween; a proof mass attached to said flexibly suspendedfirst surface; a photodiode attached to said second surface outside ofsaid cavity; a source of variable power, fixed wavelength, laser light,said light for irradiating said photodiode through said first and secondsurfaces wherein said light is also partially reflected between saidsurfaces; and an interferometer cavity length adjustor independent ofsaid photodiode and disposed outside of a path defined by said lightthrough said partially transmissive, partially reflective surfaces foradjusting said cavity length, whereby a change in said cavity length dueto movement of one of said surfaces with respect to the other of saidsurfaces is sensed by a change in said light as detected by saidphotodiode and wherein said sensitivity of said accelerometer isvariable by adjusting said cavity length, said power of said light andby activating and de-activating said light at a selected frequency. 16.The apparatus of claim 15 wherein said interferometer and saidphotodiode are monolithically integrated on a single substrate.
 17. Theapparatus of claim 15 wherein said interferometer cavity length adjustorincludes an electrode disposed so that conduction therethrough biasessaid first partially transmissive, partially reflective surface towardssaid second partially transmissive, partially reflective surface. 18.The apparatus of claim 15 wherein said interferometer cavity lengthadjustor includes an electrode disposed so that conduction therethroughbiases said first partially transmissive, partially reflective surfaceaway from said second partially transmissive, partially reflectivesurface.
 19. The apparatus of claim 17 wherein said interferometercavity length adjustor includes an electrode disposed so that conductiontherethrough biases said first partially transmissive, partiallyreflective surface away from said second partially transmissive,partially reflective surface.
 20. The apparatus of claim 17 wherein saidphotosensor is a photodiode having a substrate and wherein saidelectrode shields said first partially transmissive, partiallyreflective surface from capacitive forces exerted by said substrate. 21.The apparatus of claim 17 wherein said bias is applied between saidelectrode and said first partially transmissive, partially reflectivesurface.
 22. The apparatus of claim 17 wherein said bias is appliedbetween said electrode and a flexible support of said first partiallytransmissive, partially reflective surface.